Winter 2004 UCSC CMPE252B1 CMPE 257: Wireless and Mobile Networking SET 3p: Medium Access Control...

Winter 2004 UCSC CMPE252B

1

CMPE 257: Wireless and Mobile Networking

SET 3p:

Medium Access Control Protocols

Spring 2005 CMPE257 UCSC 2

MAC Protocol Topics Time synchronization Power saving


IEEE 802.11 Time Sync. Function Bandwidth:

Up to 54 Mbps Good for a few hundred nodes

Time Synchronization Function (TSF) Not scalable How to fix it?

Note: Only single-hop ad hoc networks are dealt with here ([HL02]).


IEEE 802.11 TSF Time divided into beacon intervals, each

containing a beacon generation window. Each station:

Waits for a random number of slots; transmits a beacon (if no one else has done

so). Beacon: several slots in length.

window

beacon interval


IEEE 802.11 TSF

Beacon contains a timestamp. On receiving a beacon, STA adopts

beacon’s timing if T(beacon) > T(STA).

Clocks move only forward.

faster adopts

12:01 12:00

slower not adopts

12:01 12:0212:01


Problems with 802.11’s TSF Faster clocks synchronize slower clocks. Equal opportunity for nodes to generate

beacons.

1:10

1:11

1:12

1:13

1:14

1:15

1:13

1:13

1:13

1:13

1:14

1:15

1:16

1:17

1:18

1:19

1:21

1:23

1:18

1:18

1:18

1:19

1:21

1:23

+3

+4

+5

+6

+7

+8

+3

+4

+5

+6

+7

+8

1:21

1:22

1:23

1:25

1:28

1:31

1:23

1:23

1:23

1:25

1:28

1:31


The Out-of-Sync Problem

When number of stations increases

Fastest station sends beacons less frequently

Stations out of synchronization


Two Types of Out-of-Sync Fastest-station out-of-sync – fastest

station is out of sync with all others. k-global out-of-sync – k percent of

links are out of sync. Questions: How often? For how

long?


Fastest-station out-of-sync (1) Clock1 and Clock2: two fastest clocks d = their difference in accuracy T = length of beacon interval (0.1 sec.) Clock drift: d*T per beacon interval. In /(d*T) intervals, fastest-station will

be out of sync with all others.

T


Fastest-station out-of-sync (2) n = number of stations. w = size of beacon

window. P’(n,w) = prob(fastest station wins beacon

contention)


Prob(Fastest station sends a beacon)


Fastest-station out-of-sync (3) H = # beacon intervals with F.S. out-

of-sync. L = # beacon intervals between

async periods. E(R) = E(H)/[E(H)+E(L)] = percent of

time in which the fastest station is out of sync with all others.

LH


How often does fastest-node get out of sync with others?


Percentage of time fastest station out of sync with all others

802.11a54 Mbps∆ = 224 s d = 0.003%


How often does 25%-async occur?


Percentage of time with 25 percent of links out-of-sync

802.11a54 Mbps∆ = 224 s d = 0.01%


How to fix it?

Desired properties: simple, efficient, and compatible with current 802.11 TSF.

Causes of out-of-sync Unidirectional clocks Equal beacon opportunity Single beacon per interval Beacon contention (collision)


Improve fastest station’s chance

Let the fastest station contend for beacon generation more frequently than others.


Adaptive Clock Sync Protocol Station x participates in beacon

contention once every C(x) intervals. Initially, C(x) =1. Always, 1 < C(x) <

Cmax. Dynamically adjust C(x):

x

faster C(x) +1x

slower C(x) -1


Once the protocol converges

Fastest station, C(x) =1

Other stations, C(x) = Cmax (Cmax= ?)


What if the fastest node leaves the IBSS?

The previously second fastest now becomes the fastest. Its C(x) will decrease to 1.


What if a new fastest node enters the IBSS?

The previously fastest now no longer the fastest. Its C(x) will increase to Cmax.


Performance 802.11 Performance of TSF ATSP Performance of ATSP TATSP Performance of Modified TSF


Modified TSF Divide stations into three groups:

Group 1: C(x) = Cmax1 = 1 Group 2: C(x) = Cmax2 = a small

number Group 3: C(x) = Cmax3 = a large

number


Performance of TSF


Performance of ATSP


Performance of Modified TSF


Summary

Showed: the IEEE 802.11 Timing Sync Function (TSF) is not scalable.

Proposed: a simple remedy compatible with the current TFS.

Choice of Cmax?


What’s Next?

IBSS: single-hop

MANET: multi-hop

transmission range


Comments Need simulations with data traffic

Some data transmissions may go beyond the Target Beacon Transmission Time (TBTT)

More realistic analysis Nodes may be still in defer state when

in beacon window time: independent, uniform assumption doesn’t hold.


Power Saving Protocols Various aspects of solution for saving

power Transmission power control Power aware routing Low-power mode

Power saving modes in IEEE 802.11 Active mode Power saving mode (PS) Protocols under Infrastructure and ad hoc

network are different


Power Saving at MAC Layer

awake sleep

Beacon window ATIM window

Beacon interval


Challenges MANET (Mobile ad hoc networks)

Multi-hop, unpredictable mobility, no plug-in power, no clock synchronization

Clock synchronization Radio interference Variable packet delay (unpredictable mobility) Lack of central control

Neighbor discovery Because PS host will reduce its transmitting/receiving

activity Routing problem

Network partitioning and merging


Design guidelines More beacon

A PS host should not inhibit its beacon in ATIM window even if it has heard other beacons

Inaccurate-neighbor problem prevention Multiple beacon in a ATIM window

Overlapping Awake interval No clock synchronization Overlapping of Wake-up pattern of two PS host

Wake-up prediction PS host’s wake-up pattern based on their time

difference


Infrastructure and Ad Hoc Protocols Access Point (AP)

monitors each host PS mode host wakes up

periodically for incoming packet from AP.

Periodic beacon frames. In each beacon frame, a

Traffic Indication Map(TIM) will be delivered, which contains ID’s of those PS host with buffered unicast packet in the AP.

PS hosts wakes up periodically

ATIM window : short interval that PS hosts wake up

In the beginning of each ATIM window, each mobile host will contend to transmit a beacon frame.

Successful beacon synchronizes mobile host’s clock and prevents other hosts from sending their beacon


Dominating-awake-interval PS host stay awake

sufficiently long so as to ensure that neighboring host can know each other.

Dominating awake property AW >= BI/2 + BW

Alternatively labeled odd and even sequence of beacon intervals


Periodically–fully-awake-interval Two types of beacon interval

Low power intervals Length of active window is reduced

to minimum Starts with an active window which

contains a beacon window followed by a MTIM window AW = BW + MW, in the rest of the time , the host can go to the sleep mode.

Fully awake intervals Length of active window is

extended to the maximum Arrives periodically, interleaved

between low power intervals AW = BI, rest of the time must

remain awake Suitable for slowly mobile

environments


Periodically–fully-awake-interval

T (=3) Beacon Interval

Beacon Window MTIM Window

Host A

Host B

Rest of active window


Properties Each PS host’s beacon window

overlaps with any neighbor’s fully-awake intervals in every T beacon intervals.

More power saving than previous protocol when T > 2.

Remark: Every node chooses the same T.


Quorum-based Quorum

A set of identities one need to obtain before doing sth. Two quorums have non-empty intersection to ensure

atomicity.

1 2 3 4

5 6 7 8

9 10 11 12

13 14 15 16

1 2 3 4

5 6 7 8

9 10 11 12

13 14 15 16

1 4 16

1 4 16


Quorum-based PS host only needs to send beacon

O(1/n) of all the beacon intervals Quorum interval

Beacon + MTIM, AW = BI Non quorum intervals

Starts with an MTIM window, after that, host may go to sleep mode, AW=MW

Amount of awaking time is less than 50%, provided n >=4

Suitable for expensive transmission cost


Communication with PS hosts

Unicast Predict PS host’s wakeup time and send

MTIM packet during that time Broadcast

Divide them into groups Hosts within the same group have

overlapping MTIM window Need multiple transmissions


Summary Three power saving protocol for

asynchronous MANETs: Dominating awake interval

Most power consumption, Lowest neighbor discovery time.

Periodically-fully-awake interval Balance both power consumption and neighbor

discovery time. Quorum based

The most power saving Longest neighbor discovery time


Comments on Simulations A custom-built simulator

Many details omitted Carrier sensing and transmission

range Star-topology Packet delivery delay (tradeoff?)


Future Work More MANET scenarios Adaptive beacon intervals?


References [HL02] Lifei Huang and Ten-Hwang

Lai, On the Scalability of IEEE 802.11 Ad Hoc Networks, in ACM MobiHoc 2002.

[THH02] Tseng et al., Power-Saving Protocols for IEEE 802.11-Based Multi-Hop Ad Hoc Networks, in IEEE INFOCOM 2002.


Acknowledgments

Parts of the presentation are adapted from the following sources: Moses Pawar, USC,

http://www.isi.edu/~weiye/teaching/cs558sm04/slides/Mac_protocols.ppt

Ten H. Lai, Ohio State University http://www.cse.ohio-state.edu/~lai/788-Au03

/2-scalibility.pdf http://www.cse.ohio-state.edu

/~lai/788-Au03/4-Power%20Saving.ppt



http://www.cse.ohio-state.edu/~lai/788-Au03/2-scalibility.pdf

http://www.cse.ohio-state.edu/~lai/788-Au03/2-scalibility.pdf

http://www.cse.ohio-state.edu/~lai/788-Au03/4-Power%20Saving.ppt

http://www.cse.ohio-state.edu/~lai/788-Au03/4-Power%20Saving.ppt

Winter 2004 UCSC CMPE252B1 CMPE 257: Wireless and Mobile Networking SET 3p: Medium Access Control...

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